1. Field of the Invention
The present invention relates to a twin wire former for the production of a fibrous material web, specifically a paper or cardboard web.
2. Description of the Related Art
A twin wire former having two rotating continuous wires which meet in the area of a simultaneously rotating dewatering element which thereby form a stock inlet gap; and a headbox from which the fiber stock suspension is fed into the stock inlet gap is described in PCT publication WO 97/47803.
The present invention provides a twin wire former which provides the highest possible web strength, soft-flake web structure, and an optimal streak-free formation.
The consistency of packaging paper and cardboard is dependent upon the volume of water which is removed on the forming roll of a former. Contrary to theoretical assumptions it has been demonstrated that a larger forming roll diameter with the same angle of wrap results in a higher dewatering capacity than a smaller diameter forming roll.
As the forming roll diameter is increased, the maximum thickness of the suspension jet exiting the headbox may also increase, without concern for backflows at the stock inlet gap.
Additionally, the achievable dimensional ratio (L/h) between jet length L and stream height h decreases with increasing suspension jet thickness which manifests itself in a streak-free formation, without significant influence of the boundary layer turbulence of the headbox walls.
The current invention creates a twin wire former of the type described at the beginning of this document with which, under consideration of the aforementioned factors the highest possible web strength, an optimum streak-free formation and an optimum soft-flake web structure is achieved.
According to the invention, the stock consistency C of the fibrous stock suspension in the headbox, as well as the basis weight F of the fibrous stock suspension supplied into the stock inlet gap is selected according to the calculation:
F/(C*1000) greater than 0.025
whereby the basis weight F is stated in g/m2 and the stock consistency C is stated in g/l.
The ratio between the maximum length of the suspension jet flowing between the discharge slice of the headbox and the stock inlet gap and the thickness of the free suspension jet is preferably smaller than 10.
If the discharge slice of the headbox is located, for example between two nozzle walls and/or one or more separating elements which are positioned transversely to the direction of the stream and one is offset from the other, then the maximum length of the free suspension jet may be determined by the distance between the set back nozzle wall and the point at which the suspension jet segment, on the side of the offset nozzle wall, impacts the relevant wire.
The dewatering element may for example be formed by a roll or by a rotating belt or fabric which would preferably be guided over curved elements. The respective roll may have an open or closed surface. It may be operated with or without vacuum. The respective belt or fabric may be specifically an open belt or fabric.
One or more formation elements and/or one or more dewatering elements may be provided following the dewatering or forming element.
In a suitable, practical embodiment of the twin wire former according to the invention, the dewatering element around which both wires wrap, has a curvature radius in the wrap-around area which is equal to or larger than approximately 900 mm and preferably larger than approximately 1000 mm. If the dewatering element is a roll, then the roll diameter is preferably greater than or equal to approximately 1800 mm, and more preferably greater than approximately 2000 mm.
It is also an advantage if the convergence angle which is formed between one of the two transverse nozzle walls and the direction of jet flow is greater than or equal to approximately 1xc2x0.
On one of the curved dewatering elements wrapped by the two wires, the ratio between the radius of curvature and the thickness of the free suspension jet is preferably less than approximately 45, and more preferably less than approximately 35.
It is also an advantage if the length of wrap X over which the two wires wrap around the dewatering element, and the thickness h of the free suspension jet are selected so that the value resulting from the equation {square root over ((X*h))} is in the range of approximately 140 mm to approximately 300 mm and preferably in a range of between approximately 160 mm and approximately 300 mm.
The wire speed v, the wire tension T and the density xcfx81 of the fiber stock suspension are appropriately designed to result in the following:       T          h      *      ρ      *              v        2               greater than   1